Contacts for semiconductor devices include bond pads deposited on the surface of the semiconductor chip, and gold wire bonds bonded to a bond pad on the device. High performance electronic hardware is characterized by high clock rate and high power. To package this class of systems, flip-chip assembly is the preferred assembly process. There are currently three major flip-chip assembly processes, all involving forming some type of contact, protruding above the bond pad of the active silicon chip (integrated circuit).
A first method employs a conventional wire bonding process. Instead of bonding to the leadframe or substrate, the bond wire is cut a 5 to 20 mils above the ball. This process usually take place after the chip is singulated from the wafer.
A second method employs a molten solder squeezed through a mask onto on a singulated chip or electroplating solder using photo-lithography techniques onto a wafer.
A third method employs an adhesive loaded with conductive particles such as gold plated metal balls or solder balls. In order to avoid shorting to adjacent conductive particles, these particles are dispersed in a controlled amount such that only one ball might contact the bond pad.
Single ball contacts, as described by the third method above, have been used. However, with the use of a single ball contact, the contact area between the ball contact and the flat contact surface of the device or circuit board to which the device is connected is a very small area, and oxides or other impurities between the ball contact and the flat contact surface of the mounting circuit board may prevent proper electrical contact between the device and the circuit board. Because of the small size of the ball, the current carrying capacity is greatly reduced.